Skip Links

USGS - science for a changing world

Scientific Investigations Report 2012–5017


Geomorphic Setting, Aquatic Habitat, and Water-Quality Conditions of the Molalla River, Oregon, 2009–10


Aquatic Habitat and Water-Quality Conditions for Fish


The Molalla River, a once highly productive salmon stream, continues to support a healthy population of benthic macroinvertebrates that feed on abundant diatoms and other algae in the river, but fish populations struggle (Oregon Department of Fish and Wildlife and National Oceanic and Atmospheric Administration, 2010). Although food resources appear adequate for fish populations, the native winter steelhead and spring Chinook salmon are challenged by several potential stressors including high water temperatures, introduced warm-water fish species, and contaminants such as pesticides that enter the lower river from tributaries draining agricultural and urban areas. Additionally, moderately high pH and low dissolved oxygen results from cycles of algal photosynthesis and benthic respiration that also may stress fish at times.


The physical habitat in the lower river includes deep holding pools for fish throughout much of the river corridor (fig. 10), but shallow-water habitat in many areas may affect salmon migration or native trout populations by contributing to elevated water temperatures that are unfavorable for fish, and by reducing available habitat. High-quality riffles suitable for salmon spawning are abundant and mostly free of fine sediment in areas exposed to flow, but increases in sand and silt in the lower river do appear to degrade riffle habitat to some degree.


Water temperatures continue to be a potentially limiting factor for fish populations, with maximum water temperatures of 22–24°C from Highway 211 Bridge to Knights Bridge in 2010. The steady warming in the study reach is caused, in part, by inputs of warm tributaries in the middle and lower reaches. Data collected by ODEQ for the temperature TMDL (Williams and Bloom, 2008) found even higher water temperatures in 2004, ranging between 24 and 26°C throughout the lower 40-km reach. TIR data collected for the TMDL found notable declines in water temperatures associated with inputs of colder tributaries, springs, seeps, and groundwater. Such areas are key habitats for fish and other aquatic life as they provide cooler “thermal refugia” such as holding pools during warm periods, and may be potential sites for future study, protection, or restoration. 


Warm water can also hamper salmonid populations by favoring warm water fish species, promoting growth of disease-causing organisms, and lowering concentrations of dissolved oxygen. Low streamflows in summer, partly a natural phenomenon due to geologic factors (lack of porous High Cascades rocks in the basin), also contributes to producing high water temperatures and may also reduce dissolved oxygen levels. Concentrations of dissolved oxygen were lowest in the early morning, with two sites—Goods and Knights Bridges—having values less than 8 mg/L in the water column. Dissolved oxygen concentrations in the gravels of fish-spawning beds in the lower river are typically about 3 mg/L lower (Carter, 2005) and could affect successful incubation of salmonid eggs. Although low inter-gravel dissolved oxygen (IGDO) concentrations in the lower Molalla River might be an issue for nonnative fall Chinook salmon, the native runs of winter steelhead and spring Chinook salmon do not use the lower river for spawning (Oregon Department of Fish and Wildlife, 1992). Nevertheless, resident salmonids in the lower river could be affected by low levels of dissolved oxygen, so future studies could characterize IGDO concentrations to determine if this is affecting native fish populations. 


Another potential water-quality limitation for fish populations and other aquatic life in the Molalla River is high pH. Although the pH is affected by geochemical factors such as the weathering of rocks, in productive rivers such as the Molalla, pH is strongly controlled by the amount of photosynthesis, which removes inorganic carbon, mostly as CO2,from the water column. Photosynthesis results in a distinct diel increase in pH during the day, and respiration by all aquatic life reintroduces CO2 causing pH to decline at night, reaching a minimum just before sunrise. Physiological effects of highly alkaline pH levels on fish may include reduced ability to excrete ammonia or regulate ion balance, and effect that is manifested within fish gills (Laurent and others, 2000). High pH can affect such physiologic processes; however, not much is known about the potential effects from a constantly changing pH. Although the pH values were within the 6.5–8.5 unit range, maximum values reached 8.4 units at Knights Bridge in September 2010 and instantaneous measurements by ODEQ were as high as 8.5 units in the lower river during past summers (fig. 27). Also, because pH and other field measurements (DO and water temperature) are typically taken in the main flow, such values may not characterize what salmonid fry, for example, experience given that they tend to congregate along stream margins, out of the main current where hydraulic flushing is less and algal abundance tends to be higher. Future studies, described below, could include continuous measurement of field parameters, or cross sectional measurements including along shorelines to better characterize the potential for negative effects on fish eggs, fry, or juveniles.


First posted February 29, 2012

For additional information contact:
Director, Oregon Water Science Center
U.S. Geological Survey
2130 SW 5th Avenue
Portland, Oregon 97201
http://or.water.usgs.gov

Part or all of this report is presented in Portable Document Format (PDF); the latest version of Adobe Reader or similar software is required to view it. Download the latest version of Adobe Reader, free of charge.

Accessibility FOIA Privacy Policies and Notices

Take Pride in America logo USA.gov logo U.S. Department of the Interior | U.S. Geological Survey
URL: http://pubsdata.usgs.gov/pubs/sir/2012/5017/section9.html
Page Contact Information: GS Pubs Web Contact
Page Last Modified: Thursday, 10-Jan-2013 19:47:34 EST